Introduction: Off Grid LPG (propane) Powered Battery Charger

Picture of Off Grid LPG (propane) Powered Battery Charger

If you live off grid, sooner or later you will have a problem with the house batteries going flat. Maybe the solar panels cant keep up because it been cloudy for a few days, or its the middle of winter and you don't have enough daylight hours. Or someone left the TV on all night, what ever the reason, chances are you will have to charge your batteries with a petrol (gasoline) generator at some stage.

Many off grids set up use a 110 or 240 volt generator plugged int a battery charge to do this, but this set up is expensive and not particularly efficient.

This Instructable covers the build of 2 two battery chargers one with a Honda clone engine and another with a Kawasaki lawn mower engine. Also shown is how to convert a car alternator to produce any voltage between 12- 60 volts DC. which is Ideal for charging the house batteries.

Also I will show you how to convert the petrol engine to LPG (propane) or natural gas, which in my part of the world is less than half the price of petrol.

Step 1: Parts and Equpment You Will Need.

Picture of Parts and Equpment You Will Need.

You will need a few parts, and you can you a second hand alternator to keep the cost down. I would recommend either buy a new engine (they are cheap enough) or use an engine that is known to be in very good condition, as a worn out engine will be unreliable and likely give problems when it is dark cold and wet....

  • 6 hp engine
  • 40 amp speed controller rated at 50 volts or more.
  • Digital Voltmeter Ammeter rated 100 volts 100 amps

  • 9 volt battery and battery snap.
  • Small push button switch
  • Automotive alternator. Rated voltage doesn't matter but the more rated amps the better.
  • Dual fuel carburetor, if you wish to run your engine on gas.
  • 5 or 6 inch pulley
  • Vee belt
  • Wires, electrical terminals, tape, heat shrink etc.
  • 25mm RHS steel to make the frame
  • Nuts bolts and fasteners
  • Hack saw, welder and general workshop tools.

Step 2: Battery Charger One. the Vertical Shaft Engine.

Picture of Battery Charger One. the Vertical Shaft Engine.

I would not recommend this approach of using a vertical shaft engine off a lawn mower, as it is more complicated to mount the motor and alternator to the frame.

However the people I made it for live in "foot rot flats" they picked up this engine for $50 and it looks to be near new.

The plate that mounts the engine and alternator was designed in CAD and cut out with a CNC plasma cutter, which is probably the hardest part of the project. A steel frame with legs made to stiffen the 6mm plate and legs were fitted to give some ground clearance and keep the belt out of the dirt.

Everything was then test fitted, and the frame was then sanded and painted and then reassembled. A pulley and belt were fitted then the contraption was almost ready to test run.

Step 3: Setting Up the Voltage and Curret Control.

Picture of Setting Up the Voltage and Curret Control.

If you are running 12 or 24 volts you can certainly wire up the appropriate voltage alternator with the original regulator and that will charge you batteries, but there is a couple of problems doing it that way.

  • First, automotive charging system are not designed to charge the battery to 100 % typically they will only charge a battery to around 80% or 14.2 volts and deep cycle battery should be charged at 14.6 - 14.8 volts
  • There is no way to control the load on you engine, a small engine will struggle to drive a large alternator if the battery is very flat.
  • 24 volt alternators can be expensive and difficult to find.
  • If you have a 36 volt or 48 volt system, its not going to work.
  • That said using the original regulator will make your charging system Idiot proof and you can buy after market regulators that will do 14.7 volts.This is what we fitted to the Honda engine charger.

To get around these problem is quite simple, just fit a motor speed controller to the rotor of the alternator. The speed controller shown in the photos was a bit over $10 and will work on 12, 24 32, or 36 volt systems. Its rated at 50 volts, so it may be damaged if been used on a 48 volt system, but Im sure there are other speed controllers you could use if you wanted to get more voltage from you charger.

To fit the speed controller you just need to..

  • Remove the alternators internal regulator (if fitted)
  • The M+ and M- on the speed controller are wired to the brushes, polarity does not matter (this is what powers the rotor)
  • The Power + and power - go to the battery been charged. Its probably worth fitting a 20 amp fuse just in case someone hooks up the batteries around the wrong way.
  • You must get the polarity correct on the power side of the speed controller.

Step 4: Fitting the Volt and Amp Meter

Picture of Fitting the Volt and Amp Meter

The volt and amp meter can be fitted next, but it didn't come with a wiring diagram. A quick internet search was able to help with that problem, and the schematic is above.

I would recommend that you power the meter from a 9 volt battery rather that the battery been charged, as it could be damaged by a voltage spike, or over voltage.

A push button can be used to give a readout which will save on the 9 volt battery life.

Step 5: Operating Procedure

Understanding how the system works and how it could be damaged, will save a lot of repairs, burnt wires and swear words. The charger is definitely not idiot proof, it can be easy damaged and over charge or flatten you battery so you need to follow the start up and shut down list.

First make sure the speed controller is turned off by turning the knob fully counterclockwise!

  1. Connect charger to the battery, double check the polarity or the speed controller will be damaged.
  2. Start the engine and set to desired rpm.
  3. Turn on the speed control by turning the knob clockwise, it will click.
  4. The speed control is very sensitive, slowly turn up the speed controller to load up the engine.
  5. The voltage and current can be checked by pushing the button.
  6. The voltage needs to be regularly checked and adjusted as necessary, there is no automatic voltage control.
  7. To shut down turn the speed control knob counterclockwise until it clicks
  8. The engine can then be shut down.

You will notice in the video the charger is easy able to over power the battery, and put out 50 amps. A car battery was used in this test, the bigger the battery the more amps you can put in without going over the battery's maximum charger voltage which is usually around 14.8 volts

Step 6: Converting to Gas

Picture of Converting to Gas

Using LPG or propane as fuel will reduce the running cost, but another advantage is that the engine oil stays clean as it not contaminated by unburnt petrol. This will increase the life of the engine considerably.

The Honda clone engines are a easy to convert as the gas carburetor was designed to bolt straight on, and work first time. The Kawasaki engine however is a completely different ball of string. Although it does kind of bolt on the engine, the throttle is on an awkward angle and the governor works back to front, compared to the Honda engine. You can see in the photos the difference between the carburetors they are almost a mirror image of each other.

A small lever was made with aluminum and glued on to the throttle with a type of super glue called rapid fix. The rod to the governor arm had to be modified and bent but the result was very good. The engines RPM is stable, the engine starts and runs as it should.

The Kawasaki engine was never fitted with a throttle that the operator could adjust, just a silly engine brake and kill switch. Its off a lawn mower, so if you let the handle go it would kill the engine and a brake would engage to stop the engine also. This nonsense was disable with a cable tie, and as the engine will only be running on gas to turn the engine off, you just turn off the gas.

Step 7: Battery Charger 2. Horizontal Shaft Engine

Picture of Battery Charger 2. Horizontal Shaft Engine

Horizontal shaft engines are much easier to set up, a simple frame can be knocked up with length of 25mm square box section. If you use four pieces laid out as shown in the photos, the belt can be can be adjusted by moving the motor. Some short legs, a mount for the alternator and a handle or two and the frame is ready for paint.

This charger had the after market regulator fitted, as the batteries are 12 volt and in poor condition. As the battery bank needs replacement the voltage can swing wildly so an automatic regulator made constant adjustment unnecessary .

This is not a problem if the battery bank is in good condition.

Step 8: Testing.

Picture of Testing.

The first test run of the Kawasaki charger caused a bit of excitement, as the students had tipped the engine over the engine oil had got inside the cylinder and exhaust, so there was clouds of thick smelly smoke which blew into several classrooms.

Needless to say the cooking class did not approve.

Once the smoke cleared the engine ran sweet and only require a small adjustment to get the engine running at the desired rpm.The Kawasaki charger gets used on a 24 volts system if there has be a day or two of rain or cloud. Its cheaper to run and way quieter than the diesel generator they had been using, and charges the batteries quicker.

The Honda engine had no such drama, and fired up on LPG on it second pull. And has been used every night for several months on a dying battery bank. Previously the owner use a petrol 240 volt generator connected to a 12volt charger and it cost him around $50 a week to run. He now spends around $30 a month on LPG. Some of the reduce cost is due to the cheaper fuel, and some of it due to the engine running a a low rpm and increase efficiency (no losses converting 240 volts to 12 volts).

The photo is of the student who built the honda charger, handing over the project to a person in need who lives without power, running water, or proper sanitation.


PatrickC268 (author)2018-01-09

if you can smell the gas you can fire the engineer

PatrickC268 (author)2018-01-09

You could always run the motor off of gas. Just like propane gas. And it wouldn't require any pressure and would be miniscule in cost compared to propane. I'm talking about regular 87 octane (lower the better) pump gasoline preferably without ethanol.

PatrickC268 (author)PatrickC2682018-01-09

This would also relieve the motor of unwanted carbon buildup which acts like the sands of time slowly wearing out nearly all internally moving parts this would also do away with a carburwaster or fuel injectowasters throw those out the window! And your oil stays nice and clean because your getting a complete burn so oil last 3-4 times longer. It keeps that pretty gold color only looses viscosity due to heat. A gallon of pump gas will typically power your average lawn mower sized engine about 2 weeks give or take a few days when sending gas to the engine as opposed to liquid gas. Funny how all these gasses get such screwy names LPG(propane) lol then you have pump gas That should be LPG what a play on words such tom foolery afoot!!! Propane is anything but liquid when its combusted! And pump gas is liquid but isn't when it combusts only the gas or gaseous part that has been converted by the heat of the motor is what is combusting. Such an immense play on words you almost run out of words to describe what is going on to the average person.

JoeF4 (author)2017-06-11

I didn't notice a mention of it, but almost all alternator cooling fans are directional. If you spin the alternator the "wrong" direction, it will still produce power, but the fan won't be pulling air across the rectifier diodes and eventually they will overheat and fail. Also, automotive V belts and pulleys are not the same as standard off the shelf v belts/pulleys.

With just slightly more work and wiring, this unit could also function as a DC welder. You'd need to wire in a switch to bypass the voltage regulation in the alternator, and a potentiometer to vary the voltage/current to the brushes to regulate the output. It's much easier to accomplish this with an externally regulated alternator.

liquidhandwash (author)JoeF42017-06-11

Thanks for the comment, Both chargers are still running without a problem, you can get both clockwise and counter clockwise fan for the bosch alternators shown above. The belts seem to be just fine, not a problem with wear or slipping.

Wide OpenL (author)2017-04-12

"makes engine last longer" but you said poor motor when i told you my car was on LPG!

Wide OpenL (author)2017-04-12

might be worth noting that nathalia shell is unmanned and you could easily fill 9kg gas cylinders from the autogas pump - i would use 45kg ones as they would be more convinient

neaton1 (author)2016-10-16

I hate to break it to you, but you can just take a 5 year old walk behind lawnmower, cut an opening into the mowing deck's top, mount an alternator to said new opening, mount a belt pulley to the output shaft, mount the belt, adjust for the slack, and lastly establish the method =for connecting the connections from said alternator to the battery(ies) you wish to charge. And for the cost of a few liters of petrol and about 30 minutes in time, you can have a battery ready to pop into the car or truck and off you go on your way.

liquidhandwash (author)neaton12016-10-16

You could do that...

shortw (author)2016-09-16

BTW, your alternator is running backwards which could heat it up even more under load since it can not cool properly.

liquidhandwash (author)shortw2016-09-19

The alternator has been used for months and it has never got hot, the only differences between a clockwise and counter clockwise alternator is the fan. In any case they are built to be inside a hot engine bay so i cant see it been a problem.

As for engine size, i dont want a small engine running a 3000 rpm or more because of the noise, and engine wear. Far better to have a larger engine running a 1200-1500 rpm at less then half the rated horse power.

JaredE (author)2016-09-11

Harbor freight has those Honda clones too. 212cc motor ~6.5hp for usually $99.

shortw (author)JaredE2016-09-16

Just go to their website and sign up to receive email and you get offers all the time. You can go to their store and sign up and you will receive coupons in the mail.

Sometimes I wonder about their quality. So cheaper may not be better.

And beware that some usable items are only available at haborfreight when you bought your machine from them. For example... I was going to buy a electric tile saw from haborfreight. First thing, the cutting blade did not come with it and they will charge for that. Second, the cutting blade that fits that tile saw can be ONLY bought at harborfreight.

Needless to say I bought my tile saw at lowes harware since the blade came with the machine and I can buy that blade that fits it anywhere. I must say that the saw was $5 more at lowes, but the blade was free.

Wingloader (author)JaredE2016-09-11

In the back of most science magazines you can get a 20% off Harbor Freight coupon. That will save you a bunch

JaredE (author)Wingloader2016-09-11

This is true and sometimes they offer a 25% off. I got mine for $94. Have had it for 1 1/2 years and never had to pull it more than 2 times to start.

You can ask around to some elderly folks if they have an AARP magazine that they are done with. They run great coupons in there.

shortw (author)2016-09-16

A 6.5 hp engine is over kill for just powering a alternator.

Look how generators and their engines are rated.

Usally it is 1kw output to 2 hp of engine.

Lets say you have an 80 amp alternator. 80 amps times 14 volt battery =1,120 watts = 1.12 kw output of that alternator times 2 = 2.24 hp needed for that gasoline engine.

So a 2.5 to 3 hp engine to be on the save side will power that alternator and that smaller engine will save on gas/gasoline since the displacement of the engine is smaller.

Just as a side note, that 80 amp alternator will charge a 800 ah battery bank at 14 volts without damaging the batteries. 13.8 volt for charging would be perfect.

tytower (author)2016-08-21

Had not thought of using a speed controller. I'll have to check in to that . I differ a bit on the battery full point . 12 Volt batteries in my experience start gassing at 14.2V. Car regulators were usually set at 13.8V . Going up so high is OK while the battery actual voltage is less than 14.2 V but I would not allow that to be exceeded as I have seen damage in the lifespan when that is allowed to happen.

I know you have heard all that before but I note it down as a consideration for readers of this excellent educational instructable for home storage battery users

liquidhandwash (author)tytower2016-08-22

Hi Tytower thanks for the feed back. The maximum voltage depends on the type of battery you have, for a flooded lead acid that is 14.8 volts and as high as 15.5 to equalize the cells. A SLA battery would be damaged with those voltages.

Check out the link

tytower (author)liquidhandwash2016-08-25

Unfortunately these days getting the truth from a battery seller is nigh
on impossible . No matter what you believe time and experience tells me
that if you go above 14.2 for any silly reason you will stuff your
battery. They want you to stuff it quickly so you buy another . I have
never heard of equalising batteries in any fashion related to lead acid
batteries . Lithium Ion , yes but lead acid -forget it it is bullshit.
lead acid cell is a normal 2.25V /cell ,They are in series so 6 make
13.5 Volts . If the cells are not equally balanced, as is always the case,
to take any one cell over 14.2, to get current to flow through them all ,
will stuff the ones with over-voltage . So to force current through at
say 15Volts probably means you are stuffing 5 cells to get the sixth up
to par. That.s why your batteries let go after a short while if you
follow their advice.


Lead Acid
The nominal voltage of lead acid is 2 volts per cell, however when measuring the open circuit voltage, the OCV of a charged and rested battery should be 2.1V/cell. Keeping lead acid much below 2.1V/cell will cause the buildup of sulfation. While on float charge, lead acid measures about 2.25V/cell, higher during normal charge.[/Quote]

Listen to experience not sales bullshit. You are a
smart man

casper10 (author)tytower2016-09-11

Hi tytower, sounds like you haven't worked on electric fork trucks,we ( my employer) had a hire fleet of them and the lead acid cells most definately needed equalizing or the cells would get all over the place and the batteries wouldn't hold the correct charge and go flat in half a shift and play up holy harry until they were equalized flattened and recharged again etc major stuff up in hire fleet, only needed one cell to be down in pack and this would happen, our fleet was equalized every weekend to keep the packs in good order, when they cost $15 grand you look after them,but you are right about the voltage when equalizing it's not 15 volts, the chargers are specially made for equallizing the batteries and cycle up & down over a longer period at lower amps & voltages until they balance then run a charge cycle not blast power in. I do the same with my motorhome charge at 2 amps for a week or every so often & they have lasted heaps. Regards casper10 also good article liquidhandwash

shortw (author)casper102016-09-16

You can not compare a trojan battery with a forklift battery. You have to boil a forklift battery or stratification of the acid will happen, since those batteries are tall.

Also forklift batteries need 20 % more power to charge compare to the same ah trojan batteries. And the fork lift batteries need higher charging voltages so you can archive the higher SOC that the lift battery manufacturer recommends so the batteries can be close enough to the falsely advertised ah.

tytower (author)casper102016-09-15

Special purpose like that yes. You can get at the individual cells . Makes all the difference . Thats what I like about Lithium Ion or LiPos they are made to access each cell . The old 12 Volt car batteries were'nt

liquidhandwash (author)tytower2016-08-26

It hard to get good info, that web site you linked in your comment also had this chart.

shortw (author)2016-09-16

I think, using the speed controller is a bad thing. You have to stay with the batteries and generator every single minute while in use to control the output of the generator because the load will change all the time which will change the amps and voltage of the generator output because the RPM of the generator is changing with the changing load.

shadowfox85 (author)2016-09-12

I recognize good ideas, like this one, I just have a problem when people try to make things fancy and say it's "off the grid." I really like this idea, hence I checked it out, and commented. Just the title of this .... is a little off.

what is it you recognize about this good idea .

I think it depends on your definition of "off grid"

Yes I agree. I love you guys and the projects you post. :)

WiggleOne (author)2016-09-11

Natural gas is 1/3 the cost of LP and it is safer and no hassle of filling tanks.

liquidhandwash (author)WiggleOne2016-09-12

The gas carb can be adjust to run on NG. They dont have a gas line near by.

Ositopuppy (author)WiggleOne2016-09-11

That's OK, if it's convenient, but not nearly as efficient as one might think. NG has less than half the energy of LP for a given volume at standard pressure. In addition, the amount of usable energy from NG is only about 85% as efficient as LP. That is, it takes more than twice the volume of gas to run your engine, and it will produce about 85% of the energy from the generator. That would make the cost differential more like: if NG cost $2 per unit, then LP costs $2.40 per unit. (It could be any denomination of currency, e.g. NG 2 Euro = LP 2,40 Euro).In my area, LP is cheaper in relation to NG, so per unit of electrical energy produced, LP would be cheaper. That...and there is no NG service in my area, so convenience isn't even a deciding factor. In your area, NG may indeed be the logical choice.

WiggleOne (author)Ositopuppy2016-09-12

Good Point. I work for a NG company so I am partial.

shadowfox85 (author)2016-09-11

Off the grid huh? Let me ask where you got the propane ... I'm fairly certain you don't mine your own.

I understand, and apologize, i took what you said wrong, no hard feelings i hope.

they guys go to wrecking yards and remove the gas cylinders from old cars, they can get a full cylinder for $10, the wrecker is happy to be rid of them.

sweet :)

efoster6 (author)2016-09-11

The problem with alternator is that when you reach 85% they drop back to a trickle charge and not the big rate at the start

liquidhandwash (author)efoster62016-09-12

Most alternators will only charge to 14.2 volts, but you can get an after market regulator to that will charge to 14.8

fred3655 (author)2016-08-21

This is slightly off topic but you may have some insight. Could an alternator be adapted to wind power easily or is the alternator so difficult to turn and/or inefficient at slower speeds & lower torque? I noticed you try to approximate the rpm from a car. Is the alternator only functional at that rotation or would the output simply be proportional to any rpm that could be generated?

colombo05 (author)fred36552016-09-11

It has to be that higher rate of speed because the internals of an alternator have no permanent magnets. It's the spinning of two separate copper coils that creates an electromagnetic field that the internal electronics will vary the current of in order to maintain a certain output voltage at a given speed.

liquidhandwash (author)fred36552016-08-21

Alternator don't make good wind turbines and they need to turn at a lest 3000 rpm and a wind turbine only turn around 500 rpm depending on size. You are much better off finding a motor from an electric tread mill, Ive made one and they work well with 800mm blades.

great info


GaryO32 (author)2016-08-20

Possibly I did not pick up on it or you did not say... But where do I purchase the LP carb if I am using a Briggs engine???

Ositopuppy (author)GaryO322016-09-11

Have you tried U.S. Carburetion? Even if a conversion kit isn't listed, give them a call. Also, they occasionally show up on Ebay. Good luck!

liquidhandwash (author)GaryO322016-08-20

Ive not seen a LP carb for a briggs, You may have to modify things to get it to fit.

Steinzel (author)2016-09-11

Excellent Instructable! Thank You for this! Very informative.

liquidsands (author)2016-09-11

This is one of the most thorough instructables I've seen. Excellent job!

itsmescotty (author)2016-08-23

30 and a few years ago I was on my 11m schooner in the Seychelles. As a live aboard charging batteries was a chore. I loathed wind turbine chargers and who could afford solar cells so that pretty much left the diesel and alternator - really quiet but the cost of diesel!!! I had a 5hp diesel driven diving compressor and thought, why not? so I acquired a 5hp or so side shaft petrol engine and bolted the spare alternator to it and 'Bob's your uncle'. Once excited I dropped it down to idle and it just quietly ticked away for hours charging batteries and practically cost nothing. Did that for years in the Indian Ocean, Red Sea. Just wished I'd known about the propane conversion. Oh yeah, I did have a petrol generator that supplied 240VAC, 120VAC and 12vDC but the petrol cost and associated noise was prohibitive.

I purchased a petrol engine yesterday to put an alternator on to charge vehicle batteries because my box charger died - how fortunate this showed up today as I have the electronics you used and will incorporate them

Thanks for the build.

fred3655 (author)2016-08-22

RE: universallvr - you would get more speed at the alternator, but you're causing more work on the engine. The engine may labor harder & consume more fuel. Every engine has an optimal load capability that doesn't stress the engine's parts & maximizes efficiency. An engine producing 6 HP equals about 4500 watts of work. Subtract about 20% for losses & that should be your peak draw.

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